Contactless cards, also known as chip cards or smart cards, operate on the basis of communication by an electromagnetic field with a read and/or write interrogating device, generically referred to as a reader.
In contactless card applications, the reader typically transmits an electromagnetic carrier wave. This transmitted carrier wave serves on the one hand to power the contactless card, which derives by induction the energy required for its operation, and on the other hand to initiate a communication between the card and the reader according to an established communication protocol. Communication protocols between a contactless card and a reader have been described, for example, in ISO standards 14443 A/B, 15693, and/or 18000.
When a plurality of contactless cards are within an interrogation field of a reader, the plurality of contactless cards will respond to a polling request of the reader. Each response signal may collide with the other cards' response signals, and therefore some cards will not be able to communicate with the reader until the reader can identify and address each card individually. Generally speaking, the reader does not know which cards are present in its interrogation field. As such, it is generally not possible for the reader to send a signal to a particular card that would activate this card while causing the others to be silent. The ISO standards describe two general types of anti-collision methods to overcome this problem: determinist methods and probabilistic methods.
According to the determinist methods, the reader sends marking commands for marking out time slots (i.e., response positions) on a time scale (i.e., temporal response scale) which includes 2N time slots. Each card sends an identification signal including its identification number when a time slot corresponding to the first N bits of its identification number is reached. When a card is the only one to respond on a time slot, the reader identifies it and can select it.
If two cards respond during the same time slot, then the first N bits of their identification number are identical. The reader detects the collision and sends a nominative complementary identification request, including the N bits of the colliding identification numbers. This nominative complementary identification request only relates to the cards colliding on the time slot defined by the N bits specified. In response to the nominative complementary identification request, the cards concerned establish a new time slot, this time using the next N bits of their identification number, and send back a new identification signal.
The above sequence can be repeated several times until all the cards are identified and/or selected. It may also be repeated for each time slot as many times as the number of sub-groups of N bits remaining to be covered in the identification numbers.
The deterministic identification process has a “tree” structure in that each nominative complementary identification step only relates to the cards colliding during a pre-determined time slot. Therefore, for example, if two first cards have N identical first bits, and two other cards have N identical first bits that are different from the N first bits of the first two cards, the identification of the four cards will require at least two complementary identification steps. That is, one step will be needed to choose between the first two cards, and the second step to choose between the second two cards.
According to the second method, so-called probabilistic methods can be distinguished from the first, determinist methods in that the time slot of a card is independent of its identification number. The reader first sends a general identification request that triggers the generation of a random number in the module. This random number determines whether the card will respond to an identification signal, and include its identification number, or wait until the next cycle. Alternatively, the random number will determine during which time slot the card will respond in.
The effectiveness of probabilistic methods, when the number of cards increases, does not depend on the length of the identification numbers, but on the staging of the time scale. Therefore, the greater the number of time slots, the more effective the probabilistic method is. In the event of a collision, the non-identified cards receive a non-nominative complementary identification request and determine a new random time slot. As the complementary identification step is not nominative, it relates to cards that have collided on different time slots, unlike the nominative identification steps of determinist methods.